The major aim of this grant is to understand the molecular mechanisms that guide neuronal growth cones to their correct targets. Growth cones are guided in part by their selective affinity for specific pathways. But growth cones are also guided from a distance by their ability to recognize their appropriate targets. Little is known about the molecules involved in guiding growth cones to their correct targets, be they intermediate targets or their final synaptic partners. Both cell surface and secreted molecules are thought to play a role in such target recognition, but to date, no bonafide target recognition molecule (i.e., a molecule that imparts target specificity) has been cloned and functionally defined. This grant contains two related projects, each involving the functional analysis of a different molecule thought to play a role in the guidance of growth cones to their targets. Both of these projects are conducted in the model system, the fruitfly Drosophila, because of the powerful genetics and transgenic technologies that enable the experimenter to either remove or add specific molecules during embryonic development. In the first project, the aim is to understand how the growth cones of commissural interneurons are guided toward the midline, an intermediate target. A genetic screen was conducted to search for the molecules (signal or receptor) involved in attracting commissural growth cones. This screen led to the identification of a new gene, commissureless, which is involved in the guidance of commissural growth cones towards the midline. In commissureless mutant embryos, commissural growth cones do not extend towards the midline, and thus no commissures form. During the tenure of this grant, the commissureless gene will be cloned and sequenced, its pattern of expression determined, and its function better defined by molecular genetic analysis. This analysis will test the hypothesis that the commissureless gene product is either the signal coming from the midline cells, or the receptor for this signal expressed by commissural growth cones. In the second project, the aim is to understand how the growth cones of motoneurons are guided toward their appropriate target muscles. A molecular screen was conducted to search for molecules (surface and/or secreted) expressed by specific muscle cells and used to attract motoneuron growth cones. This screen led to the identification of a new gene, connectin, a leucine-rich repeat family molecule which is expressed on the surface of a small subset of muscle fibers, and on the growth cones and axons of the very motoneurons that innervate these muscle fibers. During the tenure of this grant, a detailed genetic analysis of the connectin gene will test the hypothesis that connectin functions as a target recognition molecule. Preliminary genetic analysis suggests that the connectin gene does indeed function as a target recognition molecule.